No. 17 Article 3/July 15, 2005

Soybean Aphid Densities Are Increasing

The trend in soybean aphid populations has been upward since the third week in June. As I have stated more than once, we have some scouts in the field weekly assessing numbers of soybean aphids on 11 plants in each field. Some fields are selected randomly, and some are visited more than once. Both the average densities of soybean aphids per plant and the percentages of plants infested increased rather significantly from July 5-6 to July 11-12 in northern Illinois.

Table 1 shows the results of sampling for soybean aphids in five northeastern Illinois counties on July 11-12. The number of fields with at least 80% of the plants with aphids increased from 4 on July 5 and 6 (not shown in the table) to 10 on July 11 and 12. Although the average densities of soybean aphids per plant in most of the 13 fields sampled were still well below threshold (250 aphids per plant), the density in one field (McHenry 1) was very large (905 aphids per plant, with a range of 512 to ~1,500 aphids per plant) and had blown right by the threshold. Obviously, an insecticide ap-plication is warranted in this field. Also, please note that the high ends of the ranges in a couple of fields were in triple figures. All fields sampled were either in the R1 or R2 stage of development.

The heavily infested field (McHenry 1, Table 1) was the same field sampled on June 14, in which the surveyors found between 175 and 200 aphids on one plant (reported in the Bulletin, issue no. 13, June 17, 2005, "Keeping Eyes on Soybean Aphids Throughout the Midwest.") These nursery plants (a term suggested by an extension entomologist in South Dakota) seem to be important for development of fieldwide infestations, assuming natural enemies are not able to suppress the populations.

Given the current trend of increasing densities and percentages of plants infested with soybean aphids, it is quite possible that soybean fields in northern Illinois will benefit from insecticide applications within the next week or two. Soybean growers in central Illinois should escalate their vigilance for soybean aphids now. Thus far, densities of aphids in central Illinois are quite low, but let's remember what happened in 2003 when economic levels of soybean aphids occurred in August.

A strong word of caution: If you deem that an insecticide application to control soybean aphids is warranted, please be very careful with timing. Honey bees readily forage in flowering soybeans, and we need to avoid killing bees. Insecticides should be applied early in the morning or late in the afternoon/evening when bees are not present in the fields. Also, please determine the location of beekeepers in the vicinity of fields that will be treated so that they can take measures to protect their hives.

It could get busier really soon. Please take every precaution to protect the environment while protecting the yield potential of the soybeans.--Kevin Steffey

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